It is often necessary to convey fluids such as water and oil over large distances and, as such, these fluids often flow under pressure through a dedicated system of pipes. Such pipes are susceptible to leaks and it is vital that such leaks are identified and located as soon as possible to avoid excessive fluid loss and further damage to the pipe. However, typically, at least part of these systems is inaccessible being situated, for example, under ground. Therefore there is a need for a method of monitoring the fluid flow in such pipes which will allow for easy determination, and accurate location, of leaks.
When a leak occurs in a conduit carrying fluid under pressure, there will be a pressure loss at the point of the leak and a pressure wave will propagate from that point in both directions along the pipe. One known method of determining the time of arrival of a pressure wave front propagating through a fluid in a conduit is disclosed in U.S. Pat. No. 5,388,455. The disclosed method involves monitoring the absolute value of the fluid pressure in the conduit at two points. Typically, in the case of steady conditions and no leaks, owing to turbulence in the fluid the pressure measured will not be a constant but will fluctuate about an average value. Once the pressure wave front from a leak reaches the points at which the pressure is being monitored the measured pressure will decrease, although the raw data will continue to be noisy. The method according to the prior art uses a least squares fit in order to determine the point at which the average value of the pressure starts to fall. In principle, by comparing the time at which this occurs at the two points on the conduit, if the speed of propagation of the pressure waves in the fluid is known then, assuming that the source of the measured pressure wave was a leak, the position of said leak can be determined using time of flight information.
However, this method suffers from a number of problems when used to determine the presence and location of leaks. The above mentioned prior art method involves the measurement of a pressure wave front and it must be assumed that the source of said pressure wave was a leak or theft from the pipe. That is to say that other sources of pressure waves, for example, external vibration or banging of the pipe or transient waves from the packing of the pipe, may give rise to false alarms. Furthermore, the method involves the output of discrete events which correspond to wave front arrival at the point at which pressure is monitored.
Another known method uses flow meters to monitor the rate of flow of fluid at set points on the conduit. However, the output of such monitors must be integrated over a suitable time period in order to detect a flow discrepancy. As such, there is necessarily a time delay between the leak occurring and a positive determination that is has occurred.
It is an object of the present invention to at least partially overcome or alleviate the above problems.